Electrical power distribution systems are formed of a complex interconnected system of generating plants, substations, and transmission and distribution lines. Large power systems such as those in the United States and Canada are of great complexity and can be vulnerable to power disruption events that propagate through the system. To meet the increasing demand for electrical power and to reduce the vulnerability of the power systems to disturbances, it will be necessary to substantially increase transmission capacity. Building new transmission lines is both costly and time consuming, with several years generally being required to complete the land acquisition, permitting, and construction processes.
Transmission and distribution lines normally are passive systems, using designs that date back many years. Existing passive transmission systems generally are not well-suited to controlling power flow from a generating site to a particular customer. The existing systems are also subject to “loop flow,” wherein electricity flows along its path of least impedance and not along a desired contract path, which results in transmission line congestion, wheeling losses, inability to fulfill electricity supply contracts, and increased transmission loading relief (TLR) requests. Adding new generation to existing transmission networks can also cause loop flows and fault protection coordination problems, with resulting poor utilization of both generation and transmission assets.
Furthermore, AC power transmission systems are inherently subject to relatively high losses when transmitting power over very long distances—several hundred to 1,000 miles or more—which has led to the limited use of high voltage DC power transmission lines and proposals for the use of superconducting lines in the future. However, such proposals for alternatives to AC transmission lines are of uncertain practicality and, if feasible at all, would require very large investments and would not be operational for several years.
Thus, in the near term, it would be highly desirable to be able to improve the capacity of the existing AC transmission infrastructure at reasonable cost. AC transmission lines typically operate well below thermal limits because of limits imposed by reliability or stability considerations, so that existing lines could potentially carry significantly more power if non-thermal constraints could be reduced.
Several technical solutions have been proposed to increase the capacity of existing AC transmission systems. Most of these proposals relate to what is known as “Flexible AC Transmission Systems” (FACTS). Although technically viable, FACTS systems have not been commercially feasible to date because of the high cost of such systems. These high costs are due to several factors, including high power ratings (20 to 100 MVA), which require the use of high power GTO devices in custom designs, making the overall system expensive to design, build, commission and operate. High voltage ratings (as high as 345 kV) require expensive insulation and isolation requirements, and fault protection coordination requirements create high component stress, again resulting in high system cost. The proposed FACTS systems have generally involved a single installation, or at most a handful of installations, of relatively large size (similar to a substation) yielding a critical single point of failure, with resulting poor reliability and potential unavailability. As a consequence, the FACTS approach has not been implemented in a widespread commercial manner.